1. Field of the Invention
The present invention relates to an image forming apparatus for forming a developing agent image on an image carrier and, by transferring the developing agent image on a transfer material, outputting the image onto the material and, in particular, to an image forming apparatus capable of effecting a transfer output switching in accordance with the conveying position of a transfer material.
2. Description of Related Art
A printer apparatus such as an electrophotographic copying apparatus or an electrostatic recording apparatus as for example disclosed in U.S. Pat. No. 5,225,879 has a transfer charger which is a DC corona discharger and a separation charger which is an AC corona discharger. The transfer charger and the separation charger are separated from a photoconductive drum which constitutes the image-bearing member. The transfer charger discharges a back surface of a paper sheet via the DC corona to electrostatically transfer a toner image from the photoconductive drum to the paper sheet. Then the separation charger discharges the paper sheet via the AC corona to electrostatically separate the paper sheet with the toner image from the photoconductive drum.
In the conventional printer apparatus of this type, when the sheet was passed through a transfer area between the drum surface and the respective charger, the outputs of the transfer charger and separation charger were controlled at all times to given levels.
By this method, in a state in which the sheet was supported by both a transfer guide provided upstream of the transfer charger and a separation supporter provided relative to the separation charger and intimately contacted with the drum surface, it was possible to obtain a better transfer process.
In the case where a sheet left in a high humidity environment for a longer period of time is passed through the transfer area, if the leading edge portion of the sheet is passed over from the transfer guide to the separation supporter, then the leading edge portion of the sheet is separated away from the drum surface due to a decrease in stiffness of the sheet, so that there occurs a partial loss in intimate contact between the drum surface and the sheet and hence an imperfect image transfer.
Further, if such partial loss in close contact between the sheet and the drum surface occurs due to a partial separation of the trailing edge of the sheet from the transfer guide, then the trailing edge portion of the sheet moves nearer the transfer charger and separation charger. Further, when the trailing edge of the sheet leaves the transfer guide, thus no longer contacting the surface of the drum, the trailing edge portion of the sheet bends down toward the transfer charger and the separation charger. Then, the trailing edge part of the sheet vibrates, due to influence by the corona charge applied from the transfer charger and the separation charger. Consequently, the toner particles move on the trailing edge portion of the sheet, inevitably jeopardizing the transfer of a toner image. This will ultimately deteriorate the resultant image, forming while spots, for example, in the image.